In medical imaging, accurate diagnosis depends on the detection of small, low contrast details within the image. The probability of detection of these details, in turn, depends critically on their visibility in the image. A number of factors, including the object (body part) being imaged, the exposure geometry and technique, and the characteristics of the detector can affect this visibility.
In conventional screen/film radiography, for example, the output tonescale (i.e. tone reproduction function) is generally already designed into the film by the film manufacturer, and a variety of different films are available to get different "looks" depending on exam types, exposing techniques, and observer preference. In computed radiography (CR) (as described in U.S. Pat. No. Re. 31,847, reissued Mar. 12, 1985, inventor Luckey), on the other hand, the separation of image acquisition and display stages allows an image or portion of an image to be displayed at an arbitrary output contrast depending on the particular need. The ability in CR systems to adjust this output tonescale in a computer before the final image is displayed is one of the powerful features of such systems. However, choosing or creating the appropriate tonescale is not always a simple operation because it depends on, among other things, the exam type, exposure conditions, imaging modality, noise sensitivity of the image acquisition device, and dynamic range of the output device.
Commercially available computed radiography systems typically include an edge enhancement stage and then a tonescale stage. The purpose of tone scale is to define an appropriate transformation function that maps the important image information to the maximum dynamic range of the display medium (either film or soft copy, i.e., CRT display). Various methods have been proposed to generate tonescale curves for CR images. In particular, the histogram-based tonescale transformation is one of the most commonly used techniques for enhancing such images because of its efficiency. The following are exemplary of histogram-based imaging techniques for processing digital medical images: U.S. Pat. Nos. 5,068,788, issued Nov. 26, 1991, inventors Goodenough et al.; 5,198,669, issued Mar. 3, 1993, inventors Namiki, et al.; 4,839,807, issued Jun. 13, 1989, inventor Doi et al.; 5,040,225, issued Aug. 13, 1991, inventor Gouge; 4,914,295, issued Apr. 3, 1990, inventors Shimura et al.; 4,952,805, issued Aug. 28, 1990, inventor Tanaka.
Generally, such histogram-based methods work best when the peak(s) in the histogram corresponding to the undesired regions are far enough away from the peak(s) corresponding to the desired regions that they can be identified as separate. However, if the peaks overlap, which can occur if the undesired regions are nonuniformly illuminated across the image, or if scatter behind the collimators causes the signal level in that area to come close to the highly x-ray absorbing structures in the object (body part), then it becomes more difficult to separate the desired and undesired regions. Consequently, a tonescale transformation will be suboptimum if it is unduly influenced by the undesired regions of the image.
It has been proposed, in the field of digital projection radiography, to present an optimal image for viewing by a radiologist through the use of image processing techniques tying together spatial image segmentation (i.e., body part finding) and histogram analysis. (See: U.S. Pat. No. 5,164,993, issued Nov. 17, 1992, inventors Capozzi and Schaetzing; U.S. patent application Ser. No. 906,191, filed Jun. 29, 1992, inventors Jang and Schaetzing.) These techniques require that the body part be segmented from the foreground and the background and that a rule-based histogram analysis finds the appropriate histogram region of interest that corresponds to the anatomical region being imaged.
There is thus a problem in the prior art of digital radiographic imaging of providing digital image processing techniques which map the important image information to the maximum dynamic range of the display medium, and which facilitate a more accurate analysis of the image histogram.